1. Field of the Invention
The present invention relates to a coating apparatus for coating liquid materials such as a resist and developing solution.
2. Description of the Related Art
A coating apparatus is used for coating a semiconductor wafer for forming a photoresist film. In performing the coating, the wafer is fixed by vacuum suction to a supporting plate housed in a treating chamber, i.e., a spinner cup. Then, the supporting plate is rotated by a rotating mechanism. During the rotation, a liquid photoresist is dripped from a nozzle movably disposed above the supporting plate onto the wafer surface. A resist film of a desired thickness is formed on the wafer by controlling the rotation speed and time of the supporting plate while exhausting the atmosphere gas within the spinner cup.
In the conventional method described above, it is important to control accurately the wafer rotation speed and the exhaust rate of the gas from the spinner cup in order to form a resist film of a uniform thickness substantially free from micro projections. If the exhaust rate is unduly high, the flow speed of the atmosphere gas becomes excessively high around the wafer. As a result, the resist film formed is waved or striped, failing to obtain a film of a uniform thickness. On the other hand, if the exhaust rate is unduly low, the mist of photoresist thrown away from the wafer by the rotation is attached again to the wafer surface, with the result that micro projections are formed on the surface of the resist film.
To overcome these problems, it is proposed to change the gas exhaust rate in accordance with the coating operation in Unexamined Published Japanese Patent Application No. 59-82975. Also proposed are gas exhaust rate control mechanisms including the use of an exhaust gas control valve such as an electromagnetic valve, or a gas exhaust means such as a suction pump, and the use of a flap disposed in an exhaust pipe, the degree of opening of said flap being controlled (See Unexamined Published Japanese Patent Applications 58-178522; 62-102854; and 60-139363). However, the gas exhaust rate control mechanisms proposed in the past are incapable of accurately controlling the gas exhaust rate and are poor in response characteristics.